Control system for electric motor

Abstract

A control system for an electric motor comprises a current controller which produces a set of voltage demands for the motor, a current sensing means arranged to produce a current sensing output indicative of electric current in the motor, and a current signal processing means arranged to process the current sensing output signal to produce a modified current signal in which at least one harmonic component present in the current sensing output signal due to inaccuracies in the current measurement is removed or substantially reduced. The modified current signal is combined with a demanded motor current to provide the input to the current controller.

Claims

1. A control system for an electric motor, the control system comprising: a current controller which produces a set of voltage demands for the electric motor, a current sensing means arranged to produce a current sensing output signal indicative of electric current in the electric motor, current signal processing means arranged to process the current sensing output signal to produce a modified current signal in which at least one harmonic component present in the current sensing output signal due to inaccuracies in the current processing is removed or substantially reduced, and in which the modified current signal is combined with a demanded motor current to provide an input to the current controller.

2. The control system according to claim 1 in which the current sensing means is arranged in a first step to isolate the at least one harmonic component, and in a second step to compensate for the at least one harmonic component by subtracting the isolated harmonic from the current sensing output signal to produce the modified current signal.

3. The control system according to claim 2 in which the current sensing means is arranged to output a current signal in a frame of reference of the electric motor as D and Q axis components.

4. The control system according to claim 3 in which the current signal processing means performs the isolation first step by first passing the current sensing output signal through a first filter to remove a DC component of the current sending output signal and to pass the harmonic and any other frequencies, then transforms the current sensing output signal passed through the first filter into an intermediate signal in a frame of reference in which the harmonic component is a DC component, then passes the intermediate signal through a second filter to leave only the DC component, then applies a reverse transform to shift the remaining DC component back to its original harmonic frequency.

5. The control system according to claim 4 in which the first filter comprises a high pass filter that removes the DC offset, passing through the harmonic frequencies.

6. The control system according to claim 4 in which the second filter comprises a low pass filter.

7. The control system according to claim 4 in which the current signal processing means includes a synchronous filter that is arranged to transform the current sensing output signal passed through the first filter to the isolation frequency reference frame.

8. The control system according to claim 1 which separately isolates positive and negative harmonics.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a known closed loop current control system for a motor;

(2) FIG. 2 is a diagram of a closed loop current control system for a motor according to an embodiment of the invention;

(3) FIG. 3 shows the location of a single current sensor when used with a three phase PWM controlled motor.

(4) FIG. 4 is a diagram of a preferred current signal processing means (iClean) of the embodiment of FIG. 2, and

(5) FIG. 5 is a diagram of a synchronous filter that can be used in the control system of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

(6) Referring to FIG. 2 the motor 10 is controlled by a closed loop motor current control system according to an embodiment of the invention comprises a current sensing system 12 and a current controller 14. The current sensing system 12 comprises a current sensor arranged to measure the currents i.sub.A, i.sub.B, i.sub.C in the three phases of the motor, which comprise stationary windings, and output a signal indicative of the current vector in the stationary coordinates having and components. The current sensing system further comprises a coordinate transformation circuit arranged to convert the current vector from the and components in the stationary reference from, to D and Q components i.sub.D and i.sub.Q defining the current vector in the rotor reference frame, which rotates relative to the fixed windings, with the Q axis current being the torque generating component and the D axis current being non-torque-generating.

(7) The current controller 14 receives the current errorwhich as will be explained below has been cleaned to remove any harmonic components introduced as a measurement error by the current sensing means 12, and outputs a demanded voltage vector, in the form of a D and Q axis voltage demand VDQ calculated to reduce the current error so that the measured current vector approaches the demanded current vector. A further transformation circuit (not shown) receives the voltage demand from the current controller and converts it to and components V.sub. which are input to a PWM driver (also not shown) which is arranged to control a number of switches to apply voltages to the phase windings of the motor in a PWM pattern which produces the net voltage in the windings having a magnitude and direction corresponding to the voltage demand vector. The switches may be arranged in a bridge with a top and bottom switch for each motor phase as shown in FIG. 3.

(8) The current sensor in this system produces harmonic interferences because it is imperfect. Primarily these occur because the current sensor takes time to settle and the time window for taking the measurement is too short for the sensor to fully settle.

(9) The control system includes a current cleaning circuit, iClean, 16, that is arranged provide a harmonic compensation function. The compensation process that the control system is arranged to perform can be broken down into two stages: isolation of harmonics and subtraction of harmonics. Each stage may be duplicated to allow both positive and negative harmonic frequencies to be removed. The circuit is shown in detail in FIG. 4. Only the stages for the positive harmonic will be described here as the stages are functionally the same. Like reference numerals will therefore be used for parts of the circuit that deal with the positive and the negative harmonics.

(10) In a first stage, to isolate the harmonics, the current output signal in the DQ frame is first filtered by passing it through a high pass filter 22. This removes the DC component and leaves the non-DC frequencies which will include the harmonic of interest. To isolate the harmonics the current control system next uses a synchronous filter 24 that is arranged to transform the intermediate filtered signal to the isolation frequency reference frame. This allows the unwanted harmonic, which is in this case assumed to be a single harmonic of known frequency, to be isolated simply by shifting it to become a DC component.

(11) FIG. 4 shows one implementation of a synchronous filter 24, which receives as inputs the filtered current sensor output signal, the motor electrical position and the harmonic h (relative to the motor electrical frequency) to be isolated. Note that h can be positive or negative depending on whether the targeted harmonic is a positive or negative sequence component (i.e. whether it travels in the same direction as the rotor or the opposite direction).

(12) The transformation e.sup.jh performed by the filter 24 is defined as:

(13) $y\left(t\right)=e^{\mathrm{jh}}u\left(t\right)=\left[\begin{array}{cc}\cos \left(h\right)& \sin \left(h\right)\\ -\sin \left(h\right)& \cos \left(h\right)\end{array}\right]u\left(t\right)$

(14) where u(t) is the input to the filter 24, y(t) is the output, and is the electrical position of the motor.

(15) The transformed signal output by the synchronous filter 24 will contain DC and AC components. The AC components are caused by all of the other harmonics in the original signal. If the DC component is zero then the isolation frequency is not present in the current controller output voltages.

(16) The transformed signal is then passed through a low pass filter 26 that removes all other unwanted frequencies and leaves the DC component.

(17) This isolated DC component is then fed to a further synchronous filter 28 that performs a transformation which is the inverse of that performed by the synchronous filter to convert this DC correction to a sinusoidal correction signal equal to the unwanted harmonic.

(18) The filtering/transformation is performed twiceonce for positive harmonics and once for negative. The two isolated harmonics are the then fed into an adder 32 that adds them together and the resulting signal is then subtracted from the current sensor output signal in a combiner block 34. The result is that the harmonics are removed from the current sensor output signal.

(19) A comparator 18 receives the cleaned current measurement signal D and Q currents and compares them with demanded D and Q current components to generate an error. This error is fed to the PI controller.

(20) Because the harmonic errors have been removed there is no danger of them being applied to the voltage demand signal and being imposed onto the actual motor currents.

(21) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.